Measurements of electrical conductance using a tetrapolar admittance catheter are used to estimate instantaneous ventricular volume in animals and humans. The measurements of volume are plotted against ventricular pressure to determine several important parameters of cardiac physiologic function. A significant source of uncertainty in the measurement is parallel conductance due to current in the ventricular muscle. The estimated volume is larger than the blood volume alone, which is required for the diagnostic measurement. Furthermore, presently, a linear relationship between conductance and estimated volume is used to calibrate the measurements. The actual relationship is substantially nonlinear.
The invention comprises an improved method for estimating instantaneous blood volume in a ventricle by subtracting the muscle contribution form the total conductance measured. The method relies on measuring the complex admittance, rather than apparent conductance (admittance magnitude), as is presently done. Briefly, the improvement consists of measuring the phase angle in addition to admittance magnitude and then directly subtracting the muscle component from the combined measurement, thereby improving the estimate of instantaneous blood volume. The technique works because the electrical properties of muscle are frequency-dependent. while those of blood are not. We propose this calibration technique as a substantial improvement in clinical and research instrumentation calibration methods.
The invention comprises an improved method for estimating instantaneous volume of a ventricle by applying a nonlinear relationship between the measured conductance and the volume of blood in the surrounding space. The nonlinear calibration relation has been determined from experiments and numerical model studies. This calibration technique is a substantial improvement in clinical and research instrumentation calibration methods.